# Homework Help: Calculating force of impact

1. Mar 11, 2017

### Sarah

• Member warned that use of the homework template is not optional
Hey everyone so I'm having trouble figuring out this problem, it's for HW and I don't want to get an answer I just need help having it explained. I wasn't sure if I needed to use the momentum equation to figure it out.

Suppose a 1000kg car is traveling at 20 meters/second before experiencing a collision. (a) If the car runs into a steel wall and the collision occurs over the course of 0.5 seconds, then what is the resulting force of impact?

So I'm trying to figure out F right? So m=1000 kg, a= 20 m/s or do I need momentum? I'm just confused and need help breaking it down. I don't know where to plug in the 0.5 sec. The car is going from 20 m/s and is stopping but where do I put in the 0.5 sec?
Thanks!

2. Mar 11, 2017

### TSny

Welcome to PF.

Does the 20 m/s represent a velocity or an acceleration?

3. Mar 11, 2017

### Sarah

Oh! I think velocity!

4. Mar 11, 2017

### TSny

Yes.

5. Mar 11, 2017

### Sarah

would the correct formula be F x change in time = change in momentum
then J=change in momentum ?

6. Mar 11, 2017

### Sarah

and would 0.5 sec be considered the change in time?

7. Mar 11, 2017

### TSny

Yes. F is the average force of the collision.

8. Mar 11, 2017

### Sarah

so then F=ma therefore it would be (1000 kg) (change in velocity/time) = change in momentum?

9. Mar 11, 2017

### TSny

No. You're mixing the two formulas F = ma and F Δt = change in momentum.

You can just work with F Δt = change in momentum. Can you calculate the change in momentum of the car?

10. Mar 11, 2017

### Sarah

Would the change in momentum be p=MV so (1000 kg) (20 m/s) ? so 20000 kg m/s?

11. Mar 11, 2017

### TSny

So just to be clear, note that "change in momentum" means the difference between the final momentum and the initial momentum:
change in momentum = Δp = pf - pi.

What is the initial momentum?
What is the final momentum?
What is the change in momentum?

12. Mar 11, 2017

### Sarah

ok. this is getting much clearer I think...haha
ok so then the pi would = 20000 kg m/s and pf= 0 m/s therefore it would be -20000 kg m/s for the final momentum?

13. Mar 11, 2017

### TSny

Yes. Good. Except your final number is the change in momentum, right?

14. Mar 11, 2017

### Sarah

No because I'm trying to find the final force right? So where the heck does the 0.5 sec fit? change of time??

15. Mar 11, 2017

### Sarah

thank you for helping me btw

16. Mar 11, 2017

### TSny

You are trying to determine the (average) force, F, that acts on the car during the time interval, Δt, of the collision.
(Once the collision is over, there is no force. So, the final force is zero.)

The formula states that the impulse F Δt is equal to the change in momentum. Here, F is the average force during the collision (which is what you want to find). You correctly found the initial and final momenta. You are given the time interval of the collision. So, you're almost there.

17. Mar 11, 2017

### Sarah

oh! so would the formula be f 0.5 sec=-20000 kg/ms and then do the calculations to it?

18. Mar 11, 2017

### TSny

Yes.

19. Mar 11, 2017

### Sarah

yay!!! so subtract 0.5 sec to -20000?

20. Mar 11, 2017

Subtract?